Method of preparing aromatic amines



Patented Dec. 9, 1947 2,432,099 METHOD OF PREPARING AROMATIC AMINES Charles N. Kimherlin,

signor to Standard Jr., Baton Rouge, La., as- Oil Development Company,

a corporation of Delaware Application July 24, 1943, Serial No. 496,021 3 Claims. (Cl. 260-580) This invention relates to the improvements in the temperature control of highly exothermic reactions and, more particularly, it relates to a Amethod for controlling the temperature in the catalytic reduction of nitro hydrocarbons such as nitro aromatics, paraiins or naphthenes to form the corresponding amino compounds.

AThe commercial application of chemical processes involving highly exothermic reactions usually demands complicated and expensive equipment in order to gain eiective temperature control. Such temperature control is usually necessary to obtain eflicient yields of high quality products, to control reaction rate at a safe level and to avoid process run-away conditions which might exceed the maximum allowable operating conditions for the equipment used.

The present invention is particularly useful in the preparation of aromatic amines. The commercial production of these. amines has recently gained considerable prominence in the war program since these compounds when blended in small quantities with aviation gasoline materially increase the rich mixture performance of the gasoline.

When an airplane is in flight at a fairly constant rate of speed, a lean mixture of fuel and air is required for perfect combustion performance. Such a mixture contains preferably 0.065 pound of fuel per pound of air. However, it often becomes necessary for the plane to suddenly develop more power, such as when taking off from the ground, climbing suddenly other flight maneuvers.

During such times the fuel-air ratio lgreatly exceeds that desired for lean mixture performance, and in such a case, the ratio is usually from about 0.1-0.12 pound of fuel per pound of air. There has recently been developed a method in the air, and in vfor testing the rich mixture performance of an aviation gasoline. This method is known as the A. F. D.-3C test. This test determines what is commonly known as the "indicated mean effective pressure (I. M. E. P.). An aviation gasoline containing 4 cc. of lead tetraethyl per gallon,l

having an octane number of 100, and a rich mixture performance, or in other words, an` indicated mean effective pressure according to the A. F. D.' 3C test of 200 pounds per square inch, meets current specifications for 100 octane aviation gasoline. When, however, certain amino aromatic compounds such as xylidene are`added to such a 100 octane gasoline, its rich mixture performance is increased to a considerable extent. Thus, the utility of the aromatic amines as addition agents to aviation gasoline is obvious.

Reduction of aromatic nitro bodies to form the corresponding amines by catalytic hydrogenation has been demonstrated but commercial application of this process by the conventional oil hydrogenation technique is difficult because the aromatic nitro compounds are unstable at the temperatures normally used for hydrocarbon oil hydrogenation. The amines are more stable but they decompose at elevated temperatures. Furthermore, the heat of reaction is highly exothermic and considerably in excess of that encountered in other hydrogenation reactions such as the hydrogenation of diisobutylene.

A successful hydrogenation process for the production of aminesv must embody means for rigid temperature control, both from the standpoint of obtaining high'yields of high quality material as well as safeguarding personnel and equipment.

According to the present invention the aromatic amines are prepared by catalytically reducing nitro aromatic compounds under closely controlled conditions of temperature and pressure, so as to produce maximum quantities of desired amines of good quality. The hydrogenation or reduction of nitro aromatic compounds to amines is a highly exothermie reaction with a heat of reaction of about 110,000 calories per mol of nitro compound reduced. The' reaction is difi'lcult to control and run-away temperatures are encountered resulting in degradation of both feed and products.

Accordingly, the present invention has for its main object a method for controlling the temperature of highly exothermic reactions.

A more limited object of this invention is to provide a method for controlling the temperature of the hydrogenation of aromatic nitro compounds so that the large amount of heat which is released during the preparation of the aromatic amines will not'result in increasingthe temperature beyond economical limits.

According to the present invention, these and other objects are obtained by having present throughout the reaction chamber an excess of a vaporizable liquid, such as water which absorbs sensible heat with increase in temperature, but more important, will absorb latent heat and vaporize.

For illustration, one concrete embodiment of this invention is shown in the somewhat diagrammatic fiow chart comprising a single View of the accompanying drawing illustrating the use of the invention in the reduction of aromatic nitro compounds to the corresponding amines.

Referring to this drawing, liquid nitro compounds, such as nitro 'xylene isomers, from which the dinitro compounds have been removed by purifying with steam or vacuum, or a combination of the two, are fed from storage vessel l through line 2 and are pumped by pump 3 through lines 4 and 5 and mixed with hydrogen entering from line 6. The amount of hydrogen added may vary between 600 and 12,000 cu. ft. per barrel ofnitro xylene isomers. This mixture is then passed through line 1 into heat exchanger 8 where it picks up heat from products produced during the process. From this heat exchanger, the mixture of nitro compounds and hydrogen is passed through line 9 into preheater I0- where it is heated to a temperature of between 300 and 400 F. and passed through line II into the top of reactor I2.

Reactor I2 comprises a plurality of superimposed catalyst chambers diagrammatically represented as I3a, I3b, I3c and |311, having a perforated bottom, such as screens I5. These catalyst chambers are separated by spaces I4a, I4b and I4c. The catalyst in each of these chambers may be any desired hydrogenation catalyst. Because of itsgood physical strength a very suitable catalyst has been found to be -80%, preferably of molybdenum sulfide deposited on charcoal.

The mixture of heated nitro compounds and hydrogen introduced into the topmost chamber I3a immediately undergoes reaction whereby the nitro compounds are reduced by the hydrogen with the liberation of large quantities of heat. Temperatures in the reactor are maintained between 400 and 500 F., preferably about 450 F. Pressures may range from 600 to 4000 pounds per square inch. The mixture of hydrogen. reduced nitro compounds and unreduced nitro compounds is passed down through the reactor successively through each of the chambers I3b, I3c and I3d, in the latter of which the reaction is completed. Products of the reaction are withdrawn from the bottom of the reactor through line I6 and are passed through heat exchanger 8 in -heat exchange relation with the incoming mixture of hydrogen and nitro compounds being fed into the topmost catalyst chamber. The reaction products leave the heat exchanger through line I'I and are passed to cooler I 8 where they are still further reduced to a temperature at which the products condense. From cooler I8 the products pass through line I9 into separator 20. In this separator, the products are separated into gases and liquid products. The gases comprise mostly unreacted hydrogen which leave the separator through line 2I and are recycled to the system through line 6. The liquid products from the reduction reaction form two layers, the upper layer being the desired aromatic amines and the lower. layer being Water. The amines are withdrawn from separator through line 22 and passed to storage. The water forming the lower layer is withdrawn from the separator through line 23 and passed to line 26 through line 24 as hereinafter described.

As mentioned above, large quantities of heat are liberated during the reduction of the aromatic nitro compounds in reactor I2. According to this invention, the temperature in reactor I2 is controlled within a desired range by passing a portion of the nitro compounds in line 4 through line 25 where it is contacted with water entering through line 26. The amount of water added through line 2B may vary greatly, but preferably should be at least 50% of the total nitro aromatic compound fed into reactor I2 and may vary anywhere from l to 9 volumes of water per volume of nitro xylene isomers. The mixture of nitro aromatic compound and water is passed through line 21 into manifold 28 and from there distributed into reactor I2 at a plurality of points such as, for example, through lines 28a, 29h and 29e. While the mixture of water and nitro aromatic compounds may be introduced at any point along the reactor, it is preferable to introduce this mixture in the spaces between the several catalyst beds, such as for example, at I4a, I4b and I4c. The mixture of waterv and nitro aromatic compounds introduced in this manner will be thoroughly contacted with the nitro xylene isomers and hydrogen introduced into the reac tor through line I I in each of the mixing chambers I4a, I4b and I4c. If desired the water may be introduced through line 21 without having been previously mixed with nitro aromatic compounds. Under these circumstances all the nitro xylenes will be passed, through line 5 into the top of reactor I2. In the mixing chambers the water will 4vaporize and absorb the heat of the hydrogenation reaction and thus prevent the excessive temperature rises caused by the exothermic character of the reaction. As mentioned above, the water withdrawn along with the reaction products through line I6, heat exchanger 8, line I1, cooler I8 and line i9 into separator 20 is withdrawn from the separator through line 23 and recycled to the water inlet line 26 through line 24.

While a general description has been given above with reference to the reduction of nitro aromatic compounds, the invention has a broader application and may be used in connection with any exothermic reaction.

Referring specically to the invention as applied to the reduction of nitro xylene isomers, the feed stock passed to the reactor from'storage tank I is primarily a mixture of ortho, meta and para nitro xylenes. These compounds can be prepared, for example, by nitrating the xylene cut obtained by the reforming of naphtha in the presence of hydrogen. During the nitration reaction, a large amount of dinitro xylenes are produced. It is desirable to remove these dinitro compounds before passing them to the reduction process, since it has been found that the dinitro derivatives of xylene decompose very easily when heated to temperatures much above 500 F. These undesired compounds can easily be removed by steam distillation, by vacuum distilla tion, or both. The dinitro derivatives may also be reduced if the amount of nitrating acid used during the nitrating process is kept below that necessary to convert all the xylenes to mononitro derivatives.

The temperature at which the nitro xylenes are hydrogenated in reactor I2 may be varied within wide limits, for example, between 400 and 600 F. However, it is preferable to maintain a temperature between 475 and 500 F. The nitro xylenes are preferably heated to a temperature of 300-400 F. in the preheater coil, the temperature rising to the desired degree upon the initiation of the reaction. As indicated above, the reaction results in a large amount of heat being liberated. Therefore, according to this invention, by adding large quantities of water to the reactor, the water will absorb the excess heat and thus keep the temperature within desired limits. It is thus obvious that any desired temperature can be obtained by simply proportioning the quantity of water admitted to the reactor. Thus a temperature of 500 F, can be obtained by feeding 2-2.5 lbs. of water per pound of aromatic nitro compound into the reactor with the aromatic nitro xylenes, the water being in excess.

While the present invention has been described nitro aromatic compounds, it is not to be considered as limited to these compounds. The use of water as a diluent in controlling the temperature of the reaction may be applied as Well for the reduction of nitro parains, and nitro naphthenes, or their derivatives to the corresponding amines.

The nature and objects of the present invention having thus been set forth-and a specic embodiment of the same given, what is claimed as new and useful and desired to be secured by Letters Patent is:

1. The method of continuously reducing aromatic nitro compounds to the corresponding aromatic amines which comprises providing a reac- .tion zone containing a plurality of spaced beds of a catalyst consisting essentially of molybdenum sulfide on charcoal, introducing into said reaction zone a mixture of aromatic nitro compound and hydrogen, causing said mixture to ow through the reaction zone whereby at least a portion of the aromatic nitro compound is reduced to the corresponding aromatic amine, injecting a large quantity of liquid water into the reaction zone at points between the spaced catalyst beds causing the water to mix with the reactants and flow through the beds of catalyst at reaction tempera- .tures between 400 and 500 F. and pressures above the vapor pressure of Water at the reaction temperature, maintaining sumcient water in said reaction zone to absorb excess heat by vaporization of the Water, the water being in such excess that some liquid water is maintained in the reaction zone thereby maintaining substantial constant temperature in the reaction zone, recovering aromatic amines and Water from the reaction zone and recycling the water to the reaction zone at points between the spaced catalyst beds.

2. The method according to claim 1 in which the amount of water added is at least 50% by volume of the total aromatic nitro compound fed to the reaction zone. v

3. A process according to claim 1 in which the aromatic nitro compound is mono-nitro xylene and the aromatic amine is xylidene,

CHAR-LES N. KIMBERLIN, JR.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

